The following notation is consistently used throughout this text in order to facilitate easier delineation between various quantities:
1. DC--shorthand notation historically referring to Direct Current but now has wider meaning and refers to all Direct electrical quantities (current and voltage); PA1 2. AC--shorthand notation historically referring to Alternating Current but now has wider meaning and refers to all Alternating electrical quantities (current and voltage) PA1 3. The instantaneous time domain quantities are marked with lower case letters, such as i.sub.1 and v.sub.2 for current and voltage. Often these instantaneous quantities contain a DC component, which is designated with corresponding capital letters, such as I.sub.1 and V.sub.2. PA1 4. The difference between instantaneous and DC components is designated with .DELTA., hence .DELTA.i.sub.1 designates the ripple component or AC component of current i.sub.1. PA1 5. Duty ratio D of the input switch S.sub.1 is defined as D=t.sub.ON /T.sub.S where t.sub.ON is ON time of the switch, and T.sub.S is the switching period defined as T.sub.S =1/f.sub.S where f.sub.S is a constant switching frequency. Switch S.sub.1 is closed (turned ON) during DT.sub.S interval; PA1 6. Complementary duty ratio D' of the input switch S.sub.1 is defined as D'=1-D and D'T.sub.S is interval during which input switch S.sub.1 is turned OFF. PA1 a) Large output inductor AC current ripple is required (larger than twice the magnitude of the maximum DC load current) for soft-switching operation thereby increasing conduction losses; PA1 b) Soft-switching depends on the resonant inductor and thus is not effective over the whole operating duty ratio D range; PA1 c) Only partial soft-switching is achieved on the primary side of the isolated converter with high voltage devices, which for practical switching devices dominate the switching losses. PA1 Flux linkage .lambda. is the total flux linking all N turns and is .lambda.=N.PHI. where .PHI. is the flux in the magnetic core; PA1 Flux density B is the flux per unit area defined by B=.PHI./S where S is a magnetic core cross-section area. PA1 Inductance L is defined as the slope of .lambda.-i characteristic, i.e., L=.lambda./i; PA1 1. By insertion of the air-gap, the inductance value is drastically reduced. It is not uncommon to see the original un-gapped inductance L reduced by a factor of 100 to 1000 to the inductance L.sub.g with the air-gap included. In order to compensate for this loss of inductance, the switching frequency is radically increased or a much bigger core size is used, or both. PA1 2. The already small AC flux linkage excursions due to the finite and relatively low saturation flux density B.sub.SAT of 0.3 T (tesla) for ferrite material, is further significantly reduced due to the presence of the DC-bias in the core. For example, in typical applications, the DC-bias might correspond to a flux density of 0.25 T thus leaving only 0.05 T for the superimposed AC flux excursions. This in turn results in either larger core size requirements or increased switching frequency, or both. PA1 3. The waste of ferromagnetic material is even larger, since the negative part of the saturation characteristic is not utilized at all, and thus another .DELTA.B=B.sub.SAT =0.3 T is also wasted. PA1 1. Presently, switching DC-to-DC converters utilize magnetic components with a large air-gap in order to avoid saturation due to DC currents in their windings and the presence of DC flux. Large loss of inductance is compensated either by an increase of the switching frequency or by an increase of magnetic core size, or both, with consequent direct reduction of efficiency and increase of the magnetic core and consequently the converter size and weight. A converter with a new magnetic circuit is needed that will eliminate DC flux in the core and thus enable magnetics to be built on a ferromagnetic core without any air-gap and without any wasteful DC energy storage. In that case, the ferromagnetic material will be fully utilized in its ability to generate large inductances and effectively provide filtering, even with small size magnetics and at moderate switching frequencies. PA1 2. A number of soft switching methods proposed in the past, while producing beneficial zero voltage switching and reduction of switching losses, typically suffer from increase of conduction losses, or significantly higher voltage or current stresses on the devices compared to their Pulse Width Modulation (PWM) drive, thus ultimately resulting in diminished savings. Therefore, soft switching methods are needed without such detrimental side loss mechanisms. PA1 a) reduction of the losses due to the reverse recovery time of the body diode of the complementary output switch; PA1 b) limiting switching losses to hard-switching losses of the output switch only and eliminating hard-switching losses of the complementary output switch; PA1 c) the reduction of the D to D' transition from two-subintervals (present in both symmetrical and asymmetrical soft-switching of the non-isolated converter) to a single interval with a fast voltage rise on the input switch S.sub.1, thus making this transition considerably shorter than in either symmetrical or asymmetrical soft-switching of the non-isolated converter.
Over the last two decades a large number of switching DC-to-DC converters had been invented with the main objective to improve conversion efficiency and reduce the converter size. The past attempts to meet both of these objectives simultaneously have been hampered by the two main obstacles, which up to now seemed to be inherent to all switching DC-to-DC converters:
1) The large DC current bias present in the filtering inductors at either input or output of the converters (as well as the DC-bias current present in the isolation transformer of some of the isolated converters) resulted in a big size of the magnetic components, since an air-gap proportional to the DC current bias must be inserted in the AC flux path in order to prevent magnetic core saturation. This also resulted in a very inefficient use of the magnetic material, which was largely wasted. Even a relatively small air-gap on the order of 1 mm (40 mils), drastically reduces the total inductance. This loss of inductance was compensated by either an inordinately large increase of the switching frequency (hence increase of losses) or by increasing the size of the magnetic cores, or both.
2) Prior-art soft-switching methods, while helping to reduce switching losses, suffered from a number of disadvantages, which led to only partial reduction of the switching losses, and depending on the soft-switching type, any of the following efficiency reduction problems appeared, such as: